1. Field of the Invention
The present invention relates generally to mobile radio telephone Systems. More specifically, the present invention relates to systems and methods for controlling the amount of power that will be transmitted from a base station to a remote station in a communication system.
2. Description of the Related Art
It has recently become more common to use spread spectrum techniques, such as code division multiple access (CDMA) techniques, to communicate information over wireless communication systems. For example, CDMA techniques are in wide use for communications between stationary base stations and mobile cellular telephones in a cellular telephone network. In accordance with CDMA techniques, several streams of information, typically from different sources, are each encoded (or xe2x80x9cChannelizedxe2x80x9d) using a different code. These codes allow the information to be transmitted over the same frequency band (commonly referred to as a xe2x80x9cCDMA channelxe2x80x9d). Each such Channelized information stream is commonly referred to as a xe2x80x9cCode Channel.xe2x80x9d
It is presently well known that in order to minimize the amount of interference between Code Channels of a CDMA channel, the amount of power that is transmitted on each of the Code Channels must be carefully controlled. Furthermore, it is common for a single amplifier to be responsible for transmitting the entire CDMA channel. When a single amplifier is used to transmit an entire CDMA channel, the more power transmitted in one Code Channel, the less power is available to the other Code Channels. This is because there is typically a limit on the amount of total output power that such an amplifier can provide without undesirably distorting the amplified signals. For at least these reasons, it is important to properly allocate transmit power to each Code Channel in the same CDMA channel.
In one system used primarily for transmitting information at high data rates over a wireless communication link, all of the Code Channels in one direction are used to provide parallel data paths for information from a first end point to a second end point of the communication link. For example, information transmitted from a base station to one particular remote station is transmitted over all of the Code Channels. The transmission path in this direction is commonly referred to as either the xe2x80x9cForward Linkxe2x80x9d or xe2x80x9cDown Link.xe2x80x9d In such a high data rate system, each Code Channel on the Forward Link is allocated approximately the same amount of power for transmission from the base station. Furthermore, transmissions to different remote stations are time multiplexed. That is, during a first time slot, all of the Code Channels of the CDMA Channel are allocated to transmitting information to a first remote station. During a second time slot, all of the Code Channels of the CDMA Channel are allocated to transmitting information to a second remote station. Additional time slots provide communication links between the base station and other remote stations.
The data path by which information is transmitted from a particular remote station to the base station is commonly referred to either as the xe2x80x9cReverse Linkxe2x80x9d or the xe2x80x9cUp Link.xe2x80x9d In one high data rate system, the Code Channels of a Reverse Link are each allocated to different remote stations. The amount of power that is used to transmit the information on the Reverse Link must be controlled to reduce interference at the receiving base station between Code Channels of the same CDMA channel.
Accordingly, portions of each Code Channel on the Forward Link are reserved for transmitting power control information. The reserved portions of a particular Code Channel within one slot form a xe2x80x9cReverse Link Power Control (RLPC) Channel.xe2x80x9d Each such RLPC Channel on the Forward Link is associated with one remote station. The power control information that is transmitted on a particular RLPC Channel is intended to be received and used by one particular remote station to control the reverse link power transmitted by that particular remote station. The power control information assists in maintaining the output power from each remote station at a minimum level required for information to be reliably received from each remote station on the Reverse Link.
FIG. 1 is an illustration of the format of a Forward Link of one particular communication system. In the system shown in FIG. 1, a portion of each Code Channel forms a RLPC Channel over which reverse power control information is transmitted.
FIG. 1 shows the Forward Link 100 formatted in Code Channels 102. Two Code Channels 102a and 102b are explicitly shown in FIG. 1. However, in accordance with the format shown in FIG. 1, 32 Code Channels are provided on the Forward Link CDMA channel. Each Code Channel is divided into xe2x80x9cSlotsxe2x80x9d 104. In a typical system, such as the one shown in FIG. 1, each Slot 104 in the Forward Link has a predetermined duration. Each Slot is assigned to a particular remote station. In the system shown in FIG. 1, each Slot comprises 2048 xe2x80x9cChips.xe2x80x9d A Chip is defined as a duration in time that is equal to the duration of one bit of the code used to channelize the Code Channels. Each Slot 104 begins with a first data field 106 that is 464 Chips in length. A pilot field 108 follows the first data field 106. The pilot field is 96 chips in length. The pilot field 108, among other uses, allows the receiving device to synchronize to the phase of the incoming Forward Link signals (which include the pilot field 108 itself. A second data field 110 having a length of 464 Chips is then transmitted. A third data field 112 having a length of 400 Chips is transmitted next. Following the third data field 112, a power control field 114 is transmitted. The first power control field 114 has a length of 64 Chips. Next, a second pilot field 116 having a length of 96 Chips is transmitted, followed by a second power control field 118 having a length of 64 Chips. The last field in the Slot 104 is a fourth data field 120 having a length of 400 Chips.
The power control fields 114 and 118 within one Code Channel 102 form one RLPC Channel. Accordingly, the RLPC Channel is xe2x80x9cEmbeddedxe2x80x9d in the Data. Under most conditions, a determination can be made at the base station as to whether more, less, or the same amount of power needs to be transmitted over the Reverse Link transmitted from a remote station. The determination is made based on the strength of the signal received by the base station from a particular remote station.
Typically, when transmitting the Forward Link, the same amount of power is used to transmit each Code Channel in the CDMA channel. It is appropriate to transmit the Code Channels at the same power, since the Data is essentially directed to one remote station. For the purposes of this description, xe2x80x9cDataxe2x80x9d is defined as information that is provided by the communication system user, and does not include information that is transmitted between components of the system in order to manage and/or support system operations (such as overhead messages). However, since each RLPC Channel is directed to a different remote station, transmitting each RLPC Channel of the CDMA channel at the same power level means that some of the RLPC Channels will be transmitted at power levels that are either greater than or less than is required. This is because the amount of power that is required to transmit to a remote station that is closer is less than the amount of power required to transmit to a remote station that is farther away. Accordingly, it can be seen that transmitting all RLPC Channels at the same power level is undesirable for the following reason. There is an absolute maximum total amount of power that can be transmitted by all of the RLPC Channels taken together. Therefore, using more power than required for some RLPC Channels means that other RLPC Channels will get less power than might otherwise be possible if the power were allocated based on the actual requirements of each RLPC Channel rather than being allocated equally to all RLPC Channels. This could be problematic if the farthest remote station requires more power than 1/N, where N is the total number of RLPC Channels. It should be noted that the amount of power xe2x80x9crequiredxe2x80x9d to xe2x80x9creliablyxe2x80x9d transmit information, as referred to herein, is the amount of power that is needed to ensure that the information can be decoded with a predetermined error rate. The particular error rate depends upon the particular application of the disclosed method and apparatus.
However, determining the amount of power that is required by each RLPC Channel is difficult for some base stations from which transmission of RLPC information would be desirable. This can be understood from the following example. FIG. 2 is an illustration of a system including three base stations 201, 203, and 205 and four remote stations 207a-207d. Each of remote stations 207a-207d typically maintains a list (commonly referred to as the xe2x80x9cActive Setxe2x80x9d) of base stations from which the Forward Link 208 to that remote stations 207a, 207b, 207c, or 207d may originate. However, the Forward Link 208 will only originate from one of the base stations in the Active Set at any one time. The transmission paths 209 and 211 between those base stations 203 and 205, which are not transmitting the Forward Link 208 to the remote station 207a typically has different loss characteristics than the transmission path 213 between the base station 201 that is transmitting the Forward Link 208 and the remote station 207a. Since nothing is being transmitted to the remote station 207a from the other base stations 203 and 205 in the Active Set, it is not possible to characterize the loss over the Forward Links 209 and 211 between the other base stations 203 and 205, and the remote station 207a. Nonetheless, the remote station 207a will be transmitting to the other base stations 203 and 205. Therefore, it is desirable to have each base station 201, 203, and 205 in the Active Set send reverse link power control information to the remote station 207a so that the remote station will have information regarding the amount of power to send if selected to transmit.
Therefore, a need currently exists for a method and apparatus to determine the relative amount of power that should be used to transmit reverse link power control information from a base station that is in the Active Set of a remote station, but which is not transmitting a Forward Link signal to that remote station.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.
The disclosed method and apparatus determines how much power to allocate to each of a plurality of reverse link power control (RLPC) Channels to be transmitted from a base station, based upon data rate control (DRC) messages transmitted to the base station. However, since base stations transmit RLPC Channels to remote stations that have not necessarily transmitted a DRC to the transmitting base station, historical information is used to determine the quality of the Forward Link over which the RLPC is to be transmitted. It should be noted that for the purpose of this document, quality is directly proportional to the amount of power required to reliably transmit a predetermined amount of information in a predetermined amount of time with a predetermined error rate. If the history of the DRCs received indicates that the remote station to which the RLPC Channel is to be directed has not transmitted a DRC directed to that base station recently, then the base station allocates power to the RLPC Channels based upon information provided to the base station in DRCs that were received by the base station, but that were directed to other base stations. Accordingly, the base station can allocate power among the RLPC Channels without having received explicit information as to the quality of the Forward Link between the base station and every remote station intended to receive the information on the RLPC Channels.